GIS in Irrigation andWater Management - Practices of Irrigation and On-farm Water Management - page 428

11.8 Data and Databases for GIS
GIS data are handled in a database or databases, which will have special func-
tional requirements as well as the general characteristics of any standard database.
Geographical data are inherently a form of spatial data. Spatial data that pertain to
a location on the earth's surface are often termed geo-referenced data. Geographic
data are commonly characterized as having two fundamental components:
i. Attribute data : The phenomenon being reported such as a physical dimension or
class
ii. Spatial data : The spatial location of the phenomenon
Examples of a physical dimension might be the height of a forest canopy, the
population of a city, or width of a river. The class could be a land type, a vegeta-
tion type. The location is usually specified with reference to a common coordinate
system such as latitude and longitude.
A third fundamental component to geographic information is time. The time
component often is not stated explicitly, but it often is critical. Geographic infor-
mation describes a phenomenon at a location as it existed at a specific point of time.
A land cover map describes the location of different classes of land cover as they
existed at the time of data collection.
An agro-ecological zone (AEZ) is a geographical area delineated through a
unique combination of physiography, soil, and hydrological and agro-climatic char-
acteristics. Overlay of the agro-climatic inventory on the land resources inventory
(physiography and soil map) produces agro-climatic zone/region. AEZ is an effec-
tive planning tool for agricultural development purposes. Thirty agro-ecological
zones and 88 sub-zones in Bangladesh have been created which are relevant for
land use and for assessing agricultural potential.
11.9 Sources of Spatial Data
The primary data sources are remote sensing and global positioning system (GPS).
Remote Sensing : Captures digital data by means of sensors on satellite or air-
craft that provide measurement of reflectances or images of portions of the
earth. Remotely sensed data are usually in raster structure.
Global Positioning System : Allows capturing terrestrial position and vehicle
tracking, using a network of navigation satellites. Data are captured as a set
of point position readings and may be combined with other attributes of the
object by means of textual/numerical devices such as data loggers. The data
are structured as sequence of points, that is, vector format.
The spatial data is stored as X , Y coordinate pairs. Since the world is spherical
(3D), to store the geographic features in 2D system, one must project the geographic